The main cause of duck mortality is starvation on a scale not previously reported, spokeswoman Megan Gollwitzer of the New York State Department of Environmental Conservation said.

"Due to extensive ice cover and cold temperatures, diving ducks don't have access to the amount of food needed to sustain themselves," she said.

Bird watchers in western New York who have been birding since 1945 said they have never seen a die-off like this, Gollwitzer said.

Dead ducks are being observed along the near shore waters of Lake Erie and the Niagara River.

"Beginning in the second week of January, unusual numbers of dead ducks were observed in the Niagara River, and then in the open water areas of Lake Erie. The observations of dead ducks have been reported as far east as Irondequoit Bay on Lake Ontario," she said.

The majority of the birds so far have been Red-Breasted Mergansers and Greater Scaup. Other birds affected include American Coots, Lesser Scaup, Common Mergansers, Long-Tailed Ducks, White-Winged Scoters, Bufflehead, Goldeneye, Canvasback, Redheads, Pied-Billed Grebes and Horned and Red-Necked Grebes.

A Greater Scaup duck flies over an open ferry channel on Lake Champlain on Wednesday, March 5, 2014, in Essex, N.Y. The only open water within miles is attracting thousands of ducks and bald eagles looking to eat the ducks. (AP Photo/Mike Groll)

In Wisconsin, the majority of deaths have been seen in scaup and mergansers, Milwaukee County Wildlife Biologist Dianne Robinson of the Wisconsin Department of Natural Resources said.

"Extensive ice cover has pushed some diving ducks, such as red-breasted and common mergansers, greater and lesser scaups, and common goldeneyes, into deeper water where it is too deep for them to feed," Robinson said. "The problem has been primarily documented along the Lake Michigan shoreline, from Kenosha up to Green Bay."

Each year the Wisconsin DNR receives reports of a few stranded or dead ducks among this group of deep-water ducks.

"However, this winter our staff have confirmed numbers of dead birds along Lake Michigan in the hundreds of birds which likely means mortality has been in the 1,000s," Kent Van Horn, the Wisconsin waterfowl biologist, said.

"Most groups have been five to 20 dead ducks per observation, but these attract attention from people who observe them," he said. "People have also observed concentrations of dozens to hundreds of birds in small remaining pockets of open water."

Up to 200,000 waterfowl have been wintering on eastern Lake Erie and the Niagara River, with another 50,000 to 80,000 ducks in Wisconsin as well as geese and swans, Gollwitzer and Van Horn said.

Wintering waterfowl need to eat about 20 percent of their weight every day. In extreme conditions or harsh temperatures, ducks need to eat even more to sustain themselves.

"It is likely these birds have been in the process of food deprivation since early winter and are only now starting to die off in great numbers. The small pockets of open water can't provide enough food to sustain the massive concentrations of waterfowl," Gollwitzer said.

The bottom line is that in the annual variation of winter weather this winter was a harsh one, and during harsh winters, wild animals sometimes die, Van Horn said.